In many patents have already been disclosed safety devices detecting the application of a voltage to a metallic structure, the devices acting generally in parallel with a lighting arrester and providing for the control of various devices such as circuit-breakers or alarms. All of these devices have in common the fact that they use the conductivity of an element such as a diac, a photo-transistor or other voltage detector when the metallic mass voltage with respect to a neutral or the ground exceeds the operating threshold of said element, whereby the current which flows through the element which has become conductive is used for controlling a safety device such as a circuit-breaker, a ground contacting system, or any other alarm system.
The design of these prior overvoltage detectors requires the existence of a neutral or ground forming the second pole of the voltage detector, the first pole being formed of the metallic mass to be protected as such or more precisely of the live conductor with which the mass comes into contact. If the devices are easily adaptable to fixed machines to which a neutral can be connected or to machines running on rails, where the rails can be connected to ground, the solution is not applicable to mobile metallic masses normally insulated from the ground such as public works machines or similar wheeled vehicles which are, more than the fixed machines, exposed to accidental contacts with live conductors.
In FR-A-No. 2,042,404 to which correspond DE-A-No. 2,023,114 and US-A-No. 3,673,589 is described a presence detector comprising an antenna connected to ground via a very large impedance, with an intermediate point between the resistors in series the potential variation of which is used for controlling the conductibility of a field-effect transistor. This presence detector detects in fact the capacitance variation between the antenna or probe and the ground, under the capacitive effect of a mass, i.e. that of the intruder, which penetrates the space separating the armatures, antenna and ground. In addition to the fact that such a device requires, just as the hereabove safety devices, a contact with the ground, it would not be usable on a mobile machine since the capacitance value between the probe and the ground varies continuously as a function of the displacement, and this would cause a permanent alarm. In fact, this patent provides for a capacitor and resistor series circuit in order to make it insensitive to the charge variations of the antenna which could be produced by electric fields.